Partners and International Organizations
(English)
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AT, CH, DE, DK, ES, FI, GR, HU, IE, IT, NL, NO, PL, SE, UK
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Abstract
(English)
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Ethanol offers numerous advantages as a fuel for fuel cell application: high energy density, easy to store, handle and transport, perfectly miscible with water (for direct steam reforming), widely available and produced from renewable sources, implying CO2-neutral electrical power generation. Solid oxide fuel cell (SOFC) technology seems particularly adapted to internal steam reforming of ethanol (ESR) because of the compatible operation temperatures and tolerance towards CO. However, in practice, severe carbon deposition problems are observed with the state-of-the-art anode material, Ni-YSZ cermet, especially at temperatures below 800°C. In this project, the problem of carbon deposition related to the steam reforming reaction of ethanol directly in the anode (internal reforming) is addressed and alternative anode materials are investigated. Therefore, the following approach is pursued: 1) thermodynamic calculations are used to identify the operating conditions that should prevent carbon deposition; 2) catalytic runs are performed to characterize the performance of the candidate anode materials for ethanol steam reforming and to check their sensitivity to carbon deposition; 3) finally, electrochemical tests are performed in a sealed set-up. Thermodynamic calculations show that a steam-to-ethanol (STE) ratio between 3 and 4 is a good compromise that reduces both the C-deposition risk and the cell voltage losses due to steam excess. The catalytic activity of a standard Ni-YSZ anode material for ethanol steam reforming reaction (ESR) was characterised at various steam-to-carbon ratios: complete ethanol conversion and hy-drogen-to-ethanol yields of 4, as predicted by thermodynamic calculations, were only reached above 800°C. Furthermore, carbon formation occurred below 750°C, as confirmed by post-test TEM investigation. Similar tests were performed on a ceria-based catalyst, which was shown to be insensitive to carbon deposition although less active for ESR, with a H2-to-etOH yield of ~3 above 800°C. Because of its high tolerance towards C-deposition, ceria was therefore taken as the basis material for further anode development. In order to meet the conductivity and activity requirements for an anode application, an electronic conductive phase (Ni) was added to ceria by impregnation with a Ni-salt solution. Catalytic testing of Ni-CGO powder showed improved performance with full ethanol conversion above 600°C and selectivities close to equilibrium. Electrochemical characterisation of a reference Ni-YSZ anode supported cell with direct internal steam reforming of ethanol is also reported, setting a benchmark for improved anode materials. The performance limiting mechanisms were investigated by impedance spectroscopy. It was shown that the electrochemical performance of Ni-YSZ is directly related to the catalytic conversion of ethanol, i.e low at 700°C but similar to that obtained with reformate gas above 800°C. C-deposition was suspected in the Ni-rich anodic current collection layer and attributed to the presence of ethene and methane. Better electrochemical performances were obtained with the new Ni-CGO anode. However, high gas concentration resistance was observed and attributed to the non-optimised anode microstructure. Furthermore, delamination of the anode occurred with time resulting in large serial resistance. Finally, system modelling of a SOFC fed with ethanol was analysed using heat integration tools, indicating that the solutions favouring the production of methane from ethanol yield the highest electrical efficiency, but at a higher expected cost.
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